Soft tissue medical implants generally are made of a silicone medical grade elastomer shell and a filling material disposed inside of the shell. If the shell ruptures, either as a result of accidents or through crease-fold flaw or failure, the filling material escapes into the body. For this reason, if the filling material is a gel it is desirable that the gel contain only biocompatible ingredients, have a low total solids content, be excretable or metabolizable and be sterile.
Soft tissue medical implants, such as breast implants, testicular prostheses, chin, cheek, pectoral and calf implants and the like, but particularly, breast implants, have gone through a tremendous transition over the past few years as a result of various concerns, justified or not, regarding the safety of silicone gel filling materials. As a result, a number of materials have been proposed to replace the silicone gel materials which have been commonly used over the past 30 years, or longer.
For example, in U.S. Pat. No. 4,731,081 a number of materials including polyvinylpyrrolidone (PVP), polyvinyl alcohol, hydroxyethyl starch, lecithin, peanut oil, cottonseed oil, fatty acid salts and fatty acid esters have been proposed to prevent the problem of fold flaw.
U.S. Pat. No. 4,772,284 relates to a breast implant filled with collagen gel or a gel made from polyalpha amino homopolymers or random copolymers having a molecular weight in the range of 5,000 to 400,000.
U.S. Pat. No. 4,787,905 relates to a gel for a breast prosthesis consisting of a mixture of hydroxy-terminated polybutadiene resin, diundecylphthalate, polymethylenepolyphenyl isocyanate and dibutylin dilaurate catalyst which cures to form a gel.
U.S. Pat. No. 4,995,885 relates to an implant with alleged radiolucent characteristics made from biocompatible triglycerides, such as peanut oil or sunflower oil, having an effective atomic number of 5.9.
U.S. Pat. No. 5,287,857 relates to an implant with a gel filling material of water and a cellulose gelling agent, such as carboxymethylcellulose, methylcellulose, hydroxyethylcellulose, or the like. A lubricating agent additive is also proposed.
U.S. Pat. No. 5,219,360 relates to a filling material for an implant comprising a gel made of cross-linked hyaluronic acid and hylan.
No one has heretofore proposed that alginate gel may be used as a filling material. There is a significant amount of research on alginate materials, which will be discussed briefly herein. In molecular terms, alginates constitute a family of unbranched copolysaccharides of 1-4 linked .beta.-D-mannuronic acid (M) and .varies.-L-guluronic acid (G), of composition and sequence dependent on the organism or tissue form which they are isolated. In seaweed alginates, such as that isolated from Laminaria hyperborea, the two monomers are arranged in a pattern of blocks along the chain, with homopolymeric regions (termed M and G blocks) interspersed with regions of alternating structure (MG blocks).
It is the G-blocks of alginates which impart one of its most useful and important properties, namely, the ability of alginic acid to form water insoluble salts by electrostatic interactions with some multivalent ions (e.g., Ca.sup.2+, Ba.sup.2+, Al.sup.3+). Divalent cations, such as calcium, bind preferentially to G-blocks in a highly cooperative manner that has been termed the "egg-box model". Homogeneous alginate emulsions can be made using CaCl.sub.2 or other calcium salts in combination with ethanol, and phosphatidylcholine. Pronova Biopolymers, Inc. has recently begun promoting some highly purified alginates designed for certain biomedical applications. More specifically, the company produces ultra-pure grades with a very low content of protein, endotoxin, heavy metals and polyphenols. The company supplies alginates from Laminaria hyperborea, a seaweed naturally high in G content, which have been shown not to stimulate cytokine (e.g., IL-6, TNF-.varies.) production when incubated with monocytes.
George Blaine, in 1944, is credited with first illustrating that calcium/sodium alginate is resorbable in animals. Histologically, resorption usually occurs by phagocytosis in the absence of a dense inflammatory capsule, necrosis, fibrosis or scar tissue formation. Alginates are listed as non-irritants to human skin and eyes in the most recent BIBRA Toxicology Report. Extensive testing has established that alginate is safe when used in a body.
Clinically, Oliver and Blaine, in 1949, used alginate as an absorbable hemostat in various types of brain surgery. Since the 1940's alginate has been used successfully in other deep surgeries, as an intravenous plasma expander, for intraperitoneal and subcutaneous injections of various drugs, in various surgeries of the eye, ear, nose, mouth, throat and sinuses, as a hemostat for ulcers of the gastrointestinal tract, in morphine rectal suppositories and medical examinations of the colon, rectum and vagina, as wound, burn and surgical dressings, for ileostomal impressions, and even to correct ingrown toe-nails.
None of these prior art patents recognize or address most of the important technological issues which are involved in the selection of an appropriately safe and effective implant filling material. It would be desirable to provide a new filling material which is safe and effective and is an improvement over the characteristics of the prior art materials.
To date, no known filler has the optimized properties of both good aesthetics and rapid elimination of a large majority of the filling material. Some of the aforementioned materials, such as silicon gel, have good aesthetic properties. However, they also exhibit marginally acceptable elimination properties. Other fillers, such as saline, oils and low molecular weight polymers like PVP, have acceptable elimination properties while providing minimal aesthetics.
Accordingly, it is desirable to provide a new filling material which is safe and effective, and which also provides optimized and balanced aesthetic and elimination properties.